1. Field of the Invention
The present invention relates to a process for removal of nitrogen oxides and sulfur oxides from waste gases.
Several types of waste gases including flue gases and other combustion gases as well as waste gases from high temperature metallurgical processes contain noxious nitrogen oxides in such amounts that the release of said gases may involve an environmental risk.
The presence of nitrogen oxides in e.g. flue gases is due to the fact that nitrogen oxides may be produced by pyrolysis of nitrogen containing compounds present in the fuel and may moreover be formed by reactions of N.sub.2 and O.sub.2 at elevated temperature. Usually the nitrogen oxides are mainly present as NO, but also the other nitrogen oxides, especially NO.sub.2, are normally present. In the following all these nitrogen oxides are represented by the formula NO.sub.x.
Waste gases containing nitrogen oxides often contain also sulfur oxides, especially sulfur dioxide and in minor amounts sulfur trioxide, which sulfur oxides are represented by the formula SO.sub.x.
These sulfur oxides are generated by the combustion of sulfur containing fuel or they are formed in metallurgical processing.
Since it is also desired to avoid disposal of large amount of SO.sub.x to the atmosphere, there is a need for a process suitable for reducing not only the NO.sub.x content but also the SO.sub.x content of waste gases before disposal of the latter into the atmosphere.
2. Description of the Prior Art
Several methods have been suggested for simultaneous removal of NO.sub.x and SO.sub.x from waste gases. A review of such methods is presented in "Environmental Control Implications Of Generating Electric Power From Coal, 1977 Technological Status Report, Appendix G" prepared by Tennessee Valley Authority, Muscle Shoals, Ala., December 1977, reproduced by National Technical Information Service, Springfield, U.S.A.
These Prior Art processes may be divided into "wet processes" and "dry processes" dependent on whether or not the NO.sub.x is absorbed by means of an aqueous solution or suspension.
Of these two categories of processes the dry types are generally to be preferred to avoid those drawbacks of the wet systems which are caused e.g. by the insolubility of NO.sub.x in aqueous liquids, the extensive cooling of the waste gas, and the formation of sludge the disposal of which may cause troubles. Moreover equipment investments are substantially higher in wet processes than in dry processes.
The process according to the present invention is a dry type process.
The prior art dry processes although superior to the wet processes have certain drawbacks among which are requirement for ammonia or relatively expensive absorbents based on activated carbon. Moreover some of the dry processes use higher reaction temperatures than desirable. Also a drawback of the prior art dry processes is the sensitivity of these processes to particulates, e.g. fly ash, in the waste gas.
A prior art dry process which is reported as enabling a very efficient NO.sub.x removal utilizes radiation of the SO.sub.x and NO.sub.x containing waste gas with an electron beam thereby causing reactions between SO.sub.x and NO.sub.x resulting in products which may be separated from the gas by means of an electrostatic precipitator. However, this process demands large capital investments.
Besides the reduction processes in which ammonia is used to selectively reduce NO.sub.x to N.sub.2, other reductive methods have been suggested using hydrocarbons, carbon monoxide or hydrogen as reductants. Both of these methods, however, require large amounts of reductants in case the gas contains other oxidants than SO.sub.x and NO.sub.x, e.g. oxygen. This methods also operate at higher temperatures than convenient when flue gases are to be treated.
A further dry type process is based on adsorption of SO.sub.x and NO.sub.x on specially activated carbon. The adsorbed NO.sub.x is converted to NO.sub.2 as the carbon is heated to about 650.degree. C. for regeneration. This regeneration procedure, however, makes the total operation rather complex.
In U.S. Pat. No. 4,273,750 it has been illustrated that by using hydrated lime for desulfurization of flue gases in a dry process also a certain decrease of the NO.sub.x level in the flue gas takes place. According to said specification the flue gas which contained SO.sub.2 and NO.sub.x was contacted with the hydrated lime at temperatures between 316.degree. C. and 149.degree. C. The NO.sub.x removal by said process is, however, not as efficient as desired.
In a paper by Brown et al: "Interactions of stack gas sulfur and nitrogen oxides on dry sorbents" National Environmental Research Center, North Carolina, September 1973 (distributed by National Technical Information Service, Springfield) it is disclosed that the concentration of NO.sub.x and SO.sub.x decreases when a gas containing said two components is dryed by means of Drierite or molecular sieves. Drying by means of calcium chloride, however, did not seem to influence NO.sub.x and SO.sub.x concentrations. That discovery has, however, not been utilized in any industrial process for waste gas purification, probably because of complications involved in regeneration of the Drierite or molecular sieves.